Head-Mounted Displays (HMDs) have been developed for a growing number of uses in a range of applications, including military, commercial, industrial, fire-fighting, and entertainment applications. The design of these devices is particularly challenging due to factors such as variable facial geometry, expectations of reduced size, weight, and cost, the need for improved imaging, and requirements for ease of use.
In general, HMD optics must meet a number of basic requirements for viewer acceptance, including the following:                (i) eye relief or eye clearance. The eye relief range is defined based on viewer comfort and the optical configuration of the human eye itself. In practice, the distance between the last optical surface of the HMD optics and the viewer's eye is preferably above about 20 mm.        (ii) entrance pupil size. The entrance pupil requirement is based on physiological differences in viewer face structure as well as on gaze redirection during viewing. An entrance pupil size of at least about 10 mm diameter has been found to be workable.        (iii) field of view. For visual tasks of targeting and object recognition, a field of view (FOV) approaching about 50 degrees is considered to be sufficient.        
For many military and commercial applications, see-through capability is also required, allowing the viewer to have the advantages of both good scene visibility and, superimposed on the scene, a synthetic, electronically generated image. The human eye also has difficulty focusing on source objects at very short distances from the eye without causing eye strain. The near point for the unaided human eye is defined as the closest distance that the eye can focus on an object without causing eye strain. The near point for the unaided human eye is about 25 cm for an average middle aged adult. The near point is much longer for an older adult and can be as long as 100 cm. An HMD must provide imaging that allows sufficient translational distance.
Imaging by forming a large pupil has particular advantages when providing an HMD solution with a wide field of view. However, with conventional lens design approaches, forming a large pupil inherently causes other problems, since the lens design must attempt to correct for the pupil as well as for the wide field. The lens system must not only be able to correct for on-axis aberrations (chiefly spherical aberration and axial chromatic aberration), but for off-axis aberrations as well, including coma, astigmatism, field curvature, and distortion, and chromatic lateral aberrations. Therefore, conventional lens design approaches do not yield simple solutions for correcting aberrations when providing a large pupil with a wide field of view.
Another well known problem in achieving wide field of view using conventional optics systems is illumination falloff due to the Cosine Fourth Law, in which image brightness is reduced at a rate proportional to cos4 of the off-axis field angle. This effect can detract from realistic appearance of a synthetically generated image.
In practice, a number of tradeoffs are made with respect to these optical requirements and with respect to the overall requirements for reduced size and weight. For example, increasing the FOV tends to decrease image resolution. Improving distortion requires additional lenses, adding weight and bulk. Improving image quality overall requires a larger image source, with added weight, cost, and spacing requirements.
There have been a number of design solutions proposed and implemented for providing HMDs using various types of image sources. For example, liquid crystal devices (LCDs) and reflective liquid crystal on silicon (LCoS) devices have been used for generating the electronic images, as have various other types of spatial light modulators. Designs using such devices are hampered by the requirement that a separate light source or sources be provided. More recently, devices using Organic Light Emitting Diode (OLED) displays have been proposed for providing the source image for HMD devices. However, OLED display devices present a number of challenges to HMD design. A considerable magnification is required in order to provide a viewable image from the OLED device. Off-axis designs for electronic devices in general are less efficient than on-axis designs and require corrective optics due to image aberrations.
There is, then, a need for an inexpensive, compact HMD using an OLED device that provides the observer with a large field of view, sufficient eye relief and entrance pupil size, and an image with good image quality and with sufficient near-point distance for viewing.